Radio frequency interferometer design techniques, e.g. as described by Robert L. Goodwin, in "Ambiguity-Resistant Three and Four Channel Interferometers" (Naval Research Laboratory, Washington, D.C. Report 8005 Sep. 9, 1976), typically assume the same RF carrier frequency for all pulses used to resolve the phase ambiguities and locate the transmitter in angle. This assumption is always valid if monopulse techniques are used in making the phase measurements. However, monopulse measurements require a separate receiver pair and phase detector for each interferometer baseline (FIG. 1), and are therefore expensive in terms of both weight and cost.
As depicted in FIG. 1, one such system includes the separate receiver pair and phase detector and includes antennas 40, 42, 44, 46, 48 coupled to frequency preselectors 50, 52, 54, 56, 58, respectively. Receivers 60, 62 and 64, 66 form receiver pairs. The frequency preselectors 52, 54, 56, 58 are in turn coupled to RF/IF receivers 60, 62, 64, 66, 68, respectively, which are in turn coupled to phase detectors 70, 72, 74, 76, respectively. Power divider 78 splits the received signal and is coupled to an instantaneous frequency measurement device (IFM) 80 and to phase detector 76. Phase detectors 70, 72, 74 and 76 are coupled to phase storage 84. The phase difference .phi. induced by the signal direction-of-arrival is measured modulo (2.pi.) 86 by the individual phase detectors associated with each particular antenna, with antenna 40 as the reference. The measured phase is then stored in memory 84. The IFM 80 provides measurements to a frequency storage 82. The .phi. mod (2.pi.) measurements 86 are resolved at a resolve mod(2.pi.) .phi. ambiguity 101 process, and then the spatial signal angle of arrival (AOA) derived in a COS (AOA) process 102, which is performed using frequency storage 82 and the resolved mod(2.pi.) .phi. ambiguity 101. At process 103 the AOA and associated pulse data records or PDRs are provided to an active emitter file 104.
FIG. 2 illustrates a conventional approach to reducing the number of receivers and phase detectors required by using a baseline switch in the interferometer implementation. The conventional interferometer includes a plurality of antennas 210. The antennas 210 are coupled to an RF switch 200. The RF switch 200 is coupled to a phase detection section 209 which includes a preselector 220 coupled to an RF/IF receiver 222 which is coupled to a splitter or power divider 224. Splitter 224 is in turn coupled to an IFM 226 and a phase detector 230. The RF switch 200 is also coupled to a second preselector 232 which is also coupled to an RF/IF receiver 234. Both splitter 224 and RF/IF receiver 234 are coupled to the phase detector 230. The phase detector 230 is coupled to a sort on frequency process 201. The IFM 226 is coupled to a frequency storage device 240. The sort on frequency process 201 is coupled to a resolve mod (2.pi.) ambiguity 204. The resolve clusters are forwarded to process 205 angle measurement extraction 205. The extracted AOA is forwarded to a sort on AOA process 203. The frequency storage 240 provides frequency measurements to processes 201 and 205. A bit bucket 208 contains pulses from frequency agile radars which are unassociated with previously detected frequency agile emitters and end up in a pulse residue collection. The I-file 207 provides known frequency agile patterns.
Baseline switching, e.g., using an RF switch 200 (FIG. 2) to connect a single pair of receivers and phase detector 209 sequentially between interferometer antennas 210 can be used to save cost and weight. When implementing the baseline switching approach, intrapulse switching retains the monopulse performance. However intrapulse switching has drawbacks due to increased vulnerability to multipath. Multipath corrupts the trailing edge of received radar pulses, but not the leading edge; therefore it is most desirable to use phase measurements near the leading edge of a pulse. But typically the cumulative time required to sequentially sample the phase in intrapulse switching means at least some baseline phase measurements are made well back from the leading edge.
To allow phase measurements on the pulse leading edge, while still reducing the number of receivers used, interpulse switching can be employed. This alternative to monopulse switches the single receiver-pair and phase detector set between baselines to catch the leading edge of each pulse used to make the phase measurement. In this method the minimum number of pulses collected for a single emitter equals the number of interferometer baselines. But more pulses than this minimum number are typically collected. For example the intercept receiver tune strategy can be structured with dwells long enough to detect at least two pulses for the longest emitter pulse repetition interval expected.
Although interpulse switching saves weight and cost while reducing vulnerability to multipath, it introduces new problems. In particular, nonmonopulse switching makes the identification and location of frequency agile radars difficult. As used herein, nonmonopulse switching is identical to interpulse switching. Radars use frequency agility either as an electronic counter countermeasure (ECCM), or to enhance performance. As an example of performance use, many sea borne radars use frequency change every 10 ms to 100 ms to electronically steer the antenna beam. An ECCM application is frequency hopping within a bandwidth, possibly extending over 1 GHz, to reduce the vulnerability of surface-to-air missile systems to jamming.
Thus the change in transmitted frequency can be on a pulse-by-pulse or pulse-batch to pulse-batch basis with the RF carrier frequency of the pulse perturbed in either a random or preprogrammed fashion. But even if deterministic at the transmitter, the frequency change schedule is typically such that the frequency of the next pulse or pulse group cannot be reliably predicted from the frequency of the current pulse by ESM processing. A consequence of this unpredictability is that frequency hopping creates two serious problems for the intercept receiver: the inability to do pulse cluster-to-emitter association, and the inability to do precision emitter location in angle.
These problems arise because in a single frequency dwell the wideband ESM receiver collects pulses from many different emitters within tune bandwidths typically covering 500 MHz to 800 MHz. These pulses must be sorted or clustered by correctly associating the pulses with the individual radars. Hence, with both fixed-frequency and frequency-agile emitters, the interpulse switched system must augment the basic interferometer signal processing elements of resolving phase measurement ambiguities and extracting the signal angle-of-arrival or AOA measurements (101 and 102FIG. 1) with the additional processing 201 and 202 shown in FIG. 2. For the monopulse system, linking pulses to emitters can simply be done using angle measurements and angle predictions alone in 103, but in the interpulse system, location in angle (processes 204 and 205) and pulse-to-emitter association 203 through angle comparison, must be proceeded by pulse frequency clustering 201.
This preliminary frequency sort occurs first in an interpulse switched system because it must compensate for the nonmonopulse interferometer baseline phase sampling, associating all the pulses from a fixed-frequency emitter so that the ambiguity resolution and AOA estimation processes 204 and 205 can be performed on the frequency-clustered pulses. If this key measurement, frequency, is not available as a sorting parameter, e.g., if the emitter is frequency agile, the pulses from the radar will not form a cluster associated with a single emitter, and hence must bypass from the sort on frequency process 201 directly to the detect frequency agile emitters process 202. Thus emitter relative bearing cannot be measured and used in cluster association, and, indeed it is not even certain the pulses were transmitted by the same radar. To attempt to do the pulse-to-emitter association alternative processing 202 must be undertaken, beyond that required for the fixed frequency case.
But in current ESM systems there is no really robust alternative to using signal angle-of-arrival when doing pulse-emitter linking. For instance, process 202 typically attempts to use pulse width to perform the association. But, as noted above, interpulse switching as opposed to intrapulse switching is implemented to reduce the sensitivity to multipath pulse distortion effects, and these effects also corrupt and render unreliable pulse width measurement. Known emitter characteristics, such as deterministic frequency hopping patterns, stored in an Identified Emitter database, or I-file 207 are also commonly used. But many agile emitters vary their frequencies in a pseudo random manner, and even radars with deterministic patterns are likely to change those patterns at the outbreak of hostilities, i.e., to use special war modes. Therefore the pulses from frequency agile radars typically remain unassociated with the previously detected frequency agile emitter, and end up in the pulse residue collection or "bit bucket" 208.
But even if the pulses get correctly linked to the radar in process 202, and stored in the active emitter file (AEF) 212, the emitter still does not get located in angle, i.e., as indicated in the data flow portion of FIG. 2, processes 204 and 205 are not revisited after frequency agile emitter pulse association. This is a serious deficiency: emitter relative bearing is an important tactical, as well as ESM parameter. But, as noted previously, ESM interferometers are designed utilizing standard techniques requiring all the phases measured on the different interferometer baselines to be derived from signals at the same RF carrier frequency.
This same-frequency requirement arises from the ambiguity resolution process 101 and 204 needed in both monopulse and interpulse systems. The phase measured on the ith baseline d.sub.i of an interferometer is given by EQU .phi..sub.i =(2.pi.f/cd.sub.i.multidot.u).sub.mod(2.pi.) +.epsilon..sub..phi. (1)
or equivalently EQU .phi..sub.i =2.pi.f/cd.sub.i sin(AOA)-2.pi.n+.epsilon..sub..phi. (2)
where u is the direction-of-arrival or DOA unit vector i.e. the signal wave-normal, and c the speed of light. The integer n is the "ambiguity integer", and represents the number of equally possible AOA's. This number is a function of both the emitter signal frequency f and interferometer baseline length d.sub.i, satisfying the inequality ##EQU1##
If this inequality is true, n is equal to 0 and a unique value of .phi..sub.i may be associated with a single AOA. Equation 3 shows the unambiguous field-of-view, or FOV is inversely proportional to the baseline length. Practical systems also have measurement error .epsilon..sub..phi., as indicated in Equation 1, and this produces an AOA error ##EQU2##
also inversely proportional to the baseline length and emitter frequency. Equation 3 and Equation 4 establish the somewhat mutually exclusive elements that form the core problem in interferometer design: obtain an accurate angle estimate over a wide field-of-view (FOV) when increasing angle measurement resolution causes the unambiguous FOV to contract. Bishop solved this canonical problem by utilizing baselines long enough to provide the angle resolution needed, but which create ambiguous phase measurements (inequality 3 not satisfied); then resolving the ambiguity by measuring phases on multiple baselines with a special property. This special property is that the lengths of the baselines are prime integer multiples of the greatest common divisor of the antenna spacings. This special condition allows the unknown ambiguity integers to be uniquely estimated, i.e. the AOA ambiguities resolved, if all the phase measurements are made at the same frequency. The solution is done utilizing variations of the Chinese Remainder Theorem, such as those described in J. K. Wolf "The Chinese Remainder Theorem and Applications", Ch.16 in Blake and Poor, Communications and Networks, New York, 1986 (incorporated hereby reference in its entirety into this specification). However, the ambiguities cannot generally be resolved and the true emitter AOA estimated if the radar is frequency agile since the phase measurement properties which make the solution possible, that is the properties due to the relative prime integer spacings of the multiple baselines, are destroyed. The Chinese remainder theorem, which is based on doing integer arithmetic, cannot be applied.
Thus, there is a need in the art to overcome the drawbacks to pulse cluster-emitter association and the inability to do AOA estimation currently intrinsic to using interpulse switching against frequency agile radars.